In manufacturing of a semiconductor device or a flat panel display (FPD), various processes such as a film forming process, a heat treating process, a dry etching process, a cleaning process, and so forth are performed in a vacuum vessel (vacuum chamber) in presence of processing gases. To load or unload an object to be processed (for example, a semiconductor wafer, a glass substrate, etc.) into or from the vacuum vessel or the vacuum chamber without exposing it to the normal atmospheric environment, a load lock chamber whose atmosphere can be selectively changed into an atmospheric state or a depressurized state is connected to the vacuum chamber via a gate valve or via a gate valve and a vacuum transfer chamber (see, for example, Japanese Patent Laid-open Application No. H3-87386).
In general, the load lock chamber is connected to an atmospheric space via a door valve located opposite to a gate valve. After turning the load lock chamber into an atmospheric state, the door valve is opened to allow the load lock chamber to be exposed to the normal atmospheric environment. While the door valve is opened, an atmospheric transfer robot provided outside the load lock chamber loads an object to be processed into the load lock chamber and later unloads the processed object from the load lock chamber by using a transfer arm which is movable in forward and backward directions.
As mentioned above, the door valve is opened after the load lock chamber is turned into the atmospheric state. Here, to prevent external air from entering the load lock chamber, a pressure control may be executed by supplying a purge gas such as a nitrogen gas into the load lock chamber to thereby control the internal pressure of the load lock chamber at a pressure level slightly higher than an atmospheric pressure (i.e., a positive pressure). Despite the pressure control, however, it frequently happens that outside air enters the load lock chamber through the opened door valve when the transfer arm or the object is loaded or unloaded into or from the load lock chamber or due to external causes. In such a case, foreign substances including water molecules in the outside air as well as other particles therein may enter the load lock chamber. Those water molecules entering the load lock chamber usually end up being attached to the surface of the object to be processed, which, in turn will reduce the production yield thereof. Moreover, when loading the object to be processed, the presence of water molecules may cause an increase in processing time for a subsequent vacuum evacuation step, and when unloading the processed object, the water molecules may react with residual gas molecules stuck on or floating near the processed object, resulting in a generation of a noxious substance and/or an undesired reaction product. Meanwhile, when the door valve is opened after setting the internal pressure of the load lock chamber as a positive pressure, residual gas molecules floating in the load lock chamber would diffuse out to outside environment, which is not preferable in terms of protecting the environment. Here, the residual gas molecules may include un-reacted processing gas molecules stuck to the object in the vacuum processing chamber, reaction by-product gas molecules, gas molecules released as reaction products on the surface of the object, and so forth.
Moreover, conventionally, attempts have been made to install an air shower near an atmospheric side of the door valve (above the door valve) to shot a clean airflow downward to thereby blow out foreign substances attached on the surface of the object passing through a passageway of the door valve in an opened state. However, in case of using the air shower, the foreign substances removed from the surface of the object would be dispersed all around, i.e., into the load lock chamber or in the atmosphere. As a result, water molecules might enter the load lock chamber or the residual gas molecules might diffuse to the atmosphere, causing the same problems as mentioned above. Besides, the mechanism using the air shower has a limit in that it cannot remove foreign substances from a bottom surface of the object where the air stream is incapable of reaching, though it is possible to remove foreign substances from a top surface of the object exposed to the air stream.